Spatial and interannual variation in benthic community structure is driven by salinity effects on predator-prey dynamicsReef-associated Biodiversity

Christopher Harley¹, Alyssa Gehman², Iria Gimenez², Jakob Thyrring³

¹University of British Columbia, Canada - ²Hakai Institute, Canada - ³Aarhus University, Denmark

Benthic community structure is sensitive to environmental conditions, with ecological patterns often driven by a combination of direct effects mediated by physiological tolerance to a stressor and indirect effects mediated by shifting interspecific interactions. We investigated the role of salinity as a driver of benthic community structure on rocky shores within estuaries in British Columbia. On the BC central coast, local stream inputs deter sea star predators, resulting in elevated abundances of preferred prey (mussels) at the scale of tens of meters. Because the stratified hyposaline layer in these systems is not very thick, subdominant predators (dogwhelks) persist and thrive at the stream mouths where mussels are most abundant. In contrast, the salinity of the southern Strait of Georgia is dominated by a large river, the Fraser, and the depth of the hyposaline layer often exceeds the tidal amplitude (i.e., there is no respite from low salinity in the intertidal zone at high tide). As a result, both sea stars and whelks are negatively impacted by low salinity in the Strait of Georgia. Sea stars (Pisaster ochraceus) control mussel zonation in the region, but are increasing restricted to the subtidal in areas of low salinity over scales of tens of kilometers. As a result, mussels are more abundant at locations closer to the mouth of the Fraser River, and mussel bed extent is greater in years of high river discharge. Our results emphasize the importance of interspecific interactions in dictating the outcomes of spatial and temporal variation in dominant environmental drivers.